This invention relates generally to fuel control or delivery systems for an engine, more particularly to methods and apparatus for providing shutoff, overspeed protection, and directional control of a bypass flow in a fuel delivery system for a combustion engine, such as a gas turbine engine.
It is well-known in the art of fuel delivery systems for combustion engines, such as aircraft gas turbine engines, to use a fixed displacement pump, such as a vane or gear pump, to pressurize fuel prior to its metering and then subsequent delivery to the engine. The fixed displacement pump is typically sized to ensure an excess flow capacity at all possible operating conditions. The output of the pump is delivered to a fuel metering valve which, in conjunction with a bypassing, pressure regulating valve (PRV), meters the rate of fuel flow to the engine.
It is often desirable to have an overspeed limit in combustion engines, such as gas turbine engines, to reduce the risk of failures, such as mechanical overloading and/or excessive operating temperatures, that can occur when an engine exceeds its upper operating speeds. Overspeed conditions can occur in an engine for a number of reasons, such as for example, a sudden unforeseeable reduction in the engine load, a failure in the metering valve of the engine""s fuel delivery system, or an erroneous signal from an associated electronic engine control (EEC) which modulates the metering valve to an excessively high flow setting. It is known to provide an overspeed protection system that monitors a speed of the engine, such as the rotational speed of the power turbine of a gas turbine engine, and upon detection of an overspeed condition will reduce or stop the flow of fuel to the engine from the fuel delivery system. One such system that utilizes a mechanical overspeed governor to drive a valve is disclosed in U.S. Pat. No. 5,927,064 issued to Dryer et al. on Jul. 27, 1999, the entire Disclosure which is incorporated herein by reference. Further, when the engine is a critical component, such as a main gas turbine engine for powering an aircraft, it is known to check or verify that the overspeed protection system is functioning properly. In a typical gas turbine engine with a full authority digital electronic control (FADEC) control system and electronic overspeed protection, the proper function of the overspeed system is checked upon shutdown of the engine. In applications where the overspeed system places the control system in shutoff, and is used as the primary shutoff method, verification of proper function of the overspeed system is readily accomplished when the engine is shut down. However, if the overspeed system is not the primary method of shut off, its verification typically requires an additional feedback device (e.g., a switch).
Additionally, for aircraft, FAA FAR 25.1141 dictates that there be a method for indicating when a power assisted valve is open, closed, or traveling between the open and closed positions. Often this is accomplished by an electronic feedback signal from a sensor, such as an electrical switch, proximity transducer, or LVDT, associated with the shutoff valve of the fuel metering unit (FMU) while such systems may work well for their intended purpose, the addition of the sensor, and its associated electronic feedback, can add cost, complexity, and weight to the fuel delivery system.
Fuel delivery systems are also typically required to maintain a shutoff state with all electrical power removed, i.e., shutoff latching. Further, in many applications, while the fuel delivery system is in the shutoff state and the engine is windmilling, an elevated pressure must be maintained by the fuel delivery system to position remote actuators such as are common on gas turbine engines. Shutoff latching and windmill pressurization typically require relatively complex shutoff devices (e.g., latching torque motors or solenoids) and/or additional hardware (e.g., a sequence valve) in order to latch the shutoff state.
It is also known for aircraft gas turbine engine fuel systems to include one or more heat exchangers that transfer heat from various aircraft and/or engine components, such as the engine oil system, to the fuel prior to burning the fuel in the engine. In some systems, proper heat management requires that a bypass flow from the fuel metering unit be directed in one path, such as through one heat exchanger, under certain aircraft and/or engine conditions, and toward another path, such as another heat exchanger, under other aircraft and/or engine conditions. Some systems meet this requirement by providing a separate bypass directional control valve downstream of the fuel metering system that directs the bypass flow to one path or the other in response to a pressure rise in the output of a boost pump for the fuel delivery system, which is approximately a function of engine speed.
It is a primary object of the invention to provide new and improved apparatus and methods for providing shutoff, overspeed protection, and/or bypass flow directional control in a fuel delivery system that provides a desired fuel flow to an engine.
According to one aspect of the invention, a method is disclosed for providing a commanded shutdown mode, an overspeed shutoff mode, and an overspeed/shutoff test mode in a fuel delivery system for providing a desired fuel flow to an engine. The fuel delivery system includes a metering valve and a shutoff valve, with the metering valve having a shutoff state wherein the metering valve blocks fuel flow to the engine in response to a closing pressure in a modulated pressure chamber of the metering valve and a metering state wherein the valve provides a metered fuel flow to the engine in response to a modulated pressure in the modulated pressure chamber. The shutoff valve has an open state wherein the shutoff valve allows fuel flow to the engine from the metering valve and a closed state wherein the shutoff valve blocks flow to the engine from the metering valve. The method includes the steps of:
in response to an overspeed signal from the system to initiate the overspeed shutoff mode, actuating the shutoff valve to its closed state and directing the closing pressure to the modulated pressure chamber while also attempting to provide the modulated pressure to the modulated pressure chamber, the closing pressure being greater than the modulated pressure;
in response to a commanded shutdown signal from the system to initiate the commanded shutdown mode, actuating the shutoff valve to its closed state while providing the modulated pressure to the modulated pressure chamber; and
in response to a speed of the engine dropping below a selected sub-idle speed after the commanded shutdown mode has been initiated, transmitting an overspeed signal from the system and checking the state of the metering valve to determine if the metering valve is in the modulating state or the shutoff state while attempting to provide the modulated pressure to the modulating pressure chamber.
In one aspect of the invention, a valve pack is disclosed for use in a fuel delivery system for providing a desired fuel flow to an engine. The fuel delivery system includes a metering valve having a metering state where the metering valve provides a metered fuel flow to the engine in response to a pressure differential between a reference pressure chamber and a modulated pressure chamber of the valve and a shutoff state wherein the metering valve blocks fuel flow to the engine in response to a pressure differential between the reference and modulated pressure chambers. The valve pack includes a fuel inlet port to receive a metered fuel flow from the fuel metering valve, a fuel outlet port to selectively receive the metered fuel flow from the fuel inlet port to direct the metered fuel flow to the engine, an actuation pressure inlet port selectively connected to a drain to transfer an actuation fuel flow to the drain, a reference pressure inlet port to receive fuel flow at a reference pressure, a reference pressure outlet port to selectively receive the fuel flow at the reference pressure from the reference pressure inlet port to transfer the fuel flow to the modulated pressure chamber of the fuel metering valve, an overspeed activation port, and a shutoff actuation port. The valve pack has an overspeed state in response to the overspeed actuation port being placed in fluid communication with a drain, a shutoff state in response to the shutoff activation port being placed in fluid communication with a drain and flow from the overspeed activation port to the drain being blocked, and a run state in response to flow from both of the overspeed and shutoff drain ports to the drain being blocked. With the valve pack in the overspeed state, the pressure inlet port is connected to the reference pressure outlet port to direct fuel flow at the reference pressure from the reference pressure inlet port to the modulated pressure chamber of the fuel metering valve, the actuation pressure inlet port is opened to receive a fuel flow, and flow from the fuel inlet port to the fuel outlet port is blocked to shutoff the metered fuel flow to the engine. With the valve pack in the shutoff state, the actuation pressure inlet port is opened to receive a fuel flow, fuel flow from the reference pressure inlet port to the reference pressure outlet port is blocked, and fuel flow from the fuel inlet port to the fuel outlet port is blocked to shutoff the metered fuel flow to the engine. With the valve pack in the run state, the fuel inlet port is connected to the fuel outlet port to direct the metered fuel flow from the fuel inlet port to the engine, fuel flow from the reference pressure inlet port to the reference pressure outlet port is blocked, and the actuation pressure inlet port is closed.
In accordance with one aspect, the valve pack further includes a first valve spool positioned in the valve pack to block fuel flow from the fuel inlet port to the fuel outlet port with the valve pack in the overspeed state and with the valve pack in the shutoff state, and a second valve spool positioned in the valve pack to block fuel flow through the actuation pressure inlet port with the valve pack in the run state.
In accordance with another feature, the first and second valve spools are positioned in the valve pack to connect the reference pressure inlet and outlet ports with the valve pack in the overspeed state, the first valve spool is positioned in the valve pack to block fuel flow from the reference pressure inlet port to the reference pressure outlet port with the valve pack in the run state, and the second valve spool is positioned in the valve pack to block fuel flow from the reference pressure inlet port to the reference pressure outlet port with the valve pack in the shutoff state and with the valve pack in the run state.
In accordance with another aspect of the invention, a bypass direction control valve is disclosed for use in a fuel delivery system for providing a desired fuel flow to an engine. The fuel delivery system includes a metering valve chamber and a metering valve spool movable in the metering valve chamber. The bypass directional control valve includes a control pressure port to the metering valve chamber. The control pressure port is open to the metering valve chamber to receive fuel at a control pressure therefrom with the metering valve spool in a. first position, and the control pressure port is closed to the metering valve chamber by the metering valve spool with the metering valve spool in a second position. The bypass directional control valve further includes a bypass flow input port to receive a bypass flow from an upstream side of the metering valve, a first bypass flow output port to selectively receive the bypass flow from the bypass flow input port, a second bypass flow output port to selectively receive the bypass flow from the bypass flow input port, a control pressure chamber connected to the control pressure port to selectively receive fuel therefrom to vary a pressure in the control pressure chamber, and a valve spool movable between first and second positions in response to the pressure in the control pressure chamber. The first bypass flow output port is in fluid communication with the bypass flow input port to receive the bypass flow therefrom with the valve spool in the first position. The second bypass flow output port is in fluid communication with the bypass flow input port to receive the bypass flow therefrom with the valve spool in the second position.
In accordance with another aspect of the invention, a method is disclosed for controlling a bypass flow from a fuel delivery system that provides a desired fuel flow to an engine. The fuel delivery system includes a metering valve chamber and a metering valve spool movable in the metering valve chamber to meter the desired fuel flow to the engine. The method includes the steps of opening and closing a control pressure port from the metering valve chamber in response to movement of the metering valve spool in the metering valve chamber, directing the bypass flow to a first bypass flow outlet port in response to the control pressure port being open, and directing the bypass flow to a second bypass flow outlet port in response to the control pressure port being closed.
Other objects, aspects and advantages will become apparent from the following specification taken in connection with the accompanying drawings.